Method of obtaining mineral oil, comprising the steps of: providing a fluid comprising surface-modified silica particles, introducing this fluid into a mineral oil deposit via a first well in contact with this mineral oil deposit, recycling a mixture comprising the fluid and mineral oil via the first well or a further well and separating the mineral oil from the mixture, wherein the fluid comprises surface-modified nanostructured silica particles in an aqueous phase, A) the surface modified silica particles a) being mixed aluminium-silicon oxide, where the mixed oxide component is also part of the particle surface, and b) the surface modification comprises a hydrocarbon radical which is interrupted by one or more heteroatoms and comprises a silicon atom is bonded to a carbon atom of the hydrocarbon radical, the surface modification being selected from the group consisting of Si—(CH2)n—Ym—R, where Si is the Si atom which is bonded via a C atom to a hydrocarbon radical, and n=1, 2 or 3 and m=0 or 1 R is a radical which does not impart hydrophobic properties, and if m=1, is —H, —CH3, —C2H5, —OH, —OCH3, —OC2H5, —C(═O)OCH3, —C(═O)OC2H5, —O—C(═O)CH3, —O—C(═O)CH3, —O—C(═O)CH═CH2, —O—C(═O)CH═CH(CH3), —C(═O)CH3, —CH(O)CH2 and hydrolysates of —CH(O)CH2, and, if m=0, the aforementioned R radicals without —H, —CH3, —C2H5, Y=—(OCH2CH2)o—, o=5-15 or a mixture of the aforementioned R and Y radicals. c) having c1) a ratio of the proportions by weight of Al2O3/SiO2 of 0.1:99.9-5:95 c2) an average particle diameter d50 of 40-200 nm and c3) a carbon content of 3%-25% by weight and B) the fluid having d) a content of 0.01%-5% by weight of the surface modified silica particles, and e) a pH of the aqueous phase being 7≤pH≤8.5.